The long-term goal of this research is to determine the sites and mechanisms of motor learning in a voluntary movement system that depends heavily on inputs from the cerebral cortex and that uses the cortico-ponto-cerebellar pathways. Smooth pursuit eye movements meet these criteria and therefore afford a model system for revealing principles of motor learning that may generalize to learning of somatic movements. Under normal conditions, a moving target is needed to generate pursuit and the onset of target motion evokes and early eye acceleration at an amplitude related to target speed. Preliminary results have demonstrated that the eye acceleration in this early interval can be adapted to be larger or smaller if the target starts at one speed and, after 100ms, undergoes a step increase of decrease in speed. This learning takes about 30 minutes and is reversible and repeatable over the same time course. In this grant, one aim will use measurements of eye movements before and after learning to determine whether learning is also expressed in the response to a brief perturbation of target motion given during sustained pursuit. A second aim will record the expression of learning in the cerebellar cortex by following the responses of individual Purkinje cells in the floccular lobe before, during, and after learning. The third aim will use microstimulation and recording in extrastriate visual areas MT and MST. Microstimulation at an individual site before and after learning will reveal whether sites of learning are in the pathways from MT and MST to the cerebellum. Recordings will follow the responses of individual cells before, during, and after learning, to reveal whether learning can occur in the cerebral cortex. The final aim will develop a new class of pursuit model that is based on realistic visual inputs and that can reproduce the data obtained here. The results will provide insights into the normal processes of motor learning and may provide information about how to treat and reverse motor disorders, especially those that are learned through repetitive use of the same movement.